Method of making heteroepitaxial structures and devicce formed by the method

ABSTRACT

A method for making a heteroepitaxial layer. The method comprises providing a semiconductor substrate. A seed area delineated with a selective growth mask is formed on the semiconductor substrate. The seed area comprises a first material and has a linear surface dimension of less than 100 nm. A heteroepitaxial layer is grown on the seed area, the heteroepitaxial layer comprising a second material that is different from the first material. Devices made by the method are also disclosed.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/162,787, filed Oct. 17, 2018, which is a continuation of U.S. patentapplication Ser. No. 14/830,241, filed Aug. 19, 2015, now U.S. Pat. No.10,141,418, which is a divisional of U.S. patent application Ser. No.13/944,808, filed Jul. 17, 2013, now U.S. Pat. No. 9,142,400, whichclaims priority under 35 U.S.C. 119 to provisional application No.61/672,713 filed Jul. 17, 2012, all of which applications are hereinincorporated by reference in their entireties.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Contract No.EEC-0812056 awarded by the National Science Foundation (NSF) and/orcontract No. HDTRA1-11-1-0021 awarded by DTRA. The U.S. Government hascertain rights in this invention.

FIELD OF THE DISCLOSURE

The present disclosure is directed to a method for making aheteroepitaxial structure and devices formed by the method.

BACKGROUND

The integrated circuit industry has a long history of “Moore's law”scaling of silicon transistors from dimensions of over 10 microns totoday's 22 nm generation. In the current 22 nm generation the industryhas moved to a FinFET or tri-gate structure in which the gate is wrappedaround three sides of the silicon channel to provide improvedelectrostatic control of the carriers.

While further scaling is proceeding, the channel in the latest Inteltri-gate transistor is only about 20 atoms wide, so the end to scalingis clearly on the horizon. The industry has identified severaldirections for continuing the evolution of CMOS circuits. One directionthat is being actively investigated is the use of higher mobilitymaterials such as III-V semiconductors, III-N materials and Ge for thetransistor channel. Another alternative is the use of verticaltransistors with wrap-around (gate all around) geometries, again all ofthe same material classes are being investigated.

This evolution to heterostructure transistor structures requires newmanufacturing approaches. Two principle directions are beinginvestigated: wafer bonding; and heterostructure materials growth. Inwafer bonding, the non-silicon materials are grown on their conventionalsubstrates with the inclusion of a separation layer. Following thegrowth, the epitaxial material is bonded to a silicon wafer andselective etching is used to separate the original substrate. Films ofonly a few nm thickness have been transferred with this approach. Sincethe non-silicon material is grown using well established technologies,the issues of lattice mismatch and defects are largely controlled.However, this is a complex technology and is far from manufacturingworthy for the large area silicon substrates (today 300 nm diametermigrating to 450 nm diameter) used by the silicon integrated circuitindustry. Thermal expansion mismatch issues (the expansion coefficientsof the III-V materials and the Si substrate are different) remain.

Heteroepitaxial growth of different semiconductor and dielectricmaterials directly on Si(001) is another approach. The main issues aredefects associated with the lattice and thermal expansion mismatchesbetween the foreign material and the Si. For large area growths, theseissues give rise to dislocations and can cause cracking of the foreignfilm. Traditionally a thick buffer layer is grown to mitigate theseeffects and reduce the defects between the substrate and the activelayer. While there has been some success with this approach, it is notcompatible with integration on the very small scales of today's siliconintegrated circuits.

SUMMARY

An embodiment of the present disclosure is directed to a method formaking a heteroepitaxial layer. The method comprises providing asemiconductor substrate. A seed area delineated with a selective growthmask is formed on the semiconductor substrate. The seed area comprises afirst material and has a linear surface dimension of less than 100 nm. Aheteroepitaxial layer is grown on the seed area, the heteroepitaxiallayer comprising a second material that is different from the firstmaterial.

Another embodiment of the present disclosure is directed to a device.The device comprises a semiconductor substrate and a seed areadelineated with a selective growth mask on the semiconductor substrate.The seed area comprises a first material and a linear surface dimensionof less than 100. A heteroepitaxial layer is grown on the seed area, theheteroepitaxial layer comprising a second material that is differentfrom the first material.

Another embodiment of the present disclosure is directed to a method formaking a heteroepitaxial layer. The method comprises providing asemiconductor substrate. A nanostructured pedestal is formed on thesemiconductor substrate, the pedestal having a top surface and a sidesurface. The top surface forms a seed area having a linear surfacedimension that ranges from about 10 nm to about 100 nm. A selectivegrowth mask layer is provided on the top surface and side surface of thepedestal. A portion of the selective growth mask layer is removed toexpose the seed area of the pedestal. An epitaxial layer is grown on theseed area.

Still another embodiment of the present disclosure is directed to aheteroepitaxial nanostructure. The heteroepitaxial nanostructurecomprises a substrate. A pedestal is formed on the substrate, thepedestal having a top surface and a side surface. The top surfacecomprises a seed area. A heteroepitaxial layer is grown on the seed areaof the pedestal, the seed area having a linear surface dimension thatranges from about 10 nm to about 100 nm.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the present teachings, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which is incorporated in and constitutes apart of this specification, illustrates an embodiment of the presentteachings and together with the description, serves to explain theprinciples of the present teachings.

FIGS. 1A-E illustrate a method for making a heteroepitaxial layer,according to an embodiment of the present disclosure.

FIGS. 2A-2F illustrate a method for making heteroepitaxial layers,according to another embodiment of the present disclosure.

FIG. 3 illustrates a top view of a mask employed in an embodiment of thepresent disclosure.

FIG. 4 illustrates a device structure comprising heteroepitaxially grownlayers, according to an embodiment of the present disclosure.

FIGS. 5A-5B illustrate a schematic view of growth mask directly on a 2Dsubstrate surface, according to an embodiment of the present disclosure.

FIGS. 6A-6B illustrate a schematic view of growth mask flush withsilicon pedestals, according to an embodiment of the present disclosure.

FIGS. 7A-7B illustrate a schematic view of a growth mask extendingbeyond silicon pedestals, according to an embodiment of the presentdisclosure.

FIGS. 8A-8B illustrate a schematic view of silicon pedestals extendingbeyond growth mask, according to an embodiment of the presentdisclosure.

FIGS. 9A-9B illustrate a schematic view of non-faceted silicon pedestalsextending beyond growth mask, according to an embodiment of the presentdisclosure.

FIG. 10 illustrates an isometric projection of a heterostructure FinFETshowing an isolation layer, the source, gate and drain contacts labeledS, G and D respectively, according to an embodiment of the presentdisclosure.

FIG. 11 illustrates an isometric projection of a heterstructure FinFETwith a dielectric isolation (shown as semi-transparent for clarity),according to an embodiment of the present disclosure.

FIG. 12 illustrates a layered heterostructure for channel region of adevice, according to an embodiment of the present disclosure.

FIGS. 13-14 illustrate schematic views of vertical nanowire transistorsgrown seed areas, according to an embodiment of the present disclosure.

It should be noted that some details of the figure have been simplifiedand are drawn to facilitate understanding of the embodiments rather thanto maintain strict structural accuracy, detail, and scale.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the presentteachings, examples of which are illustrated in the accompanyingdrawing. In the drawings, like reference numerals have been usedthroughout to designate identical elements. In the followingdescription, reference is made to the accompanying drawing that forms apart thereof, and in which is shown by way of illustration a specificexemplary embodiment in which the present teachings may be practiced.The following description is, therefore, merely exemplary.

The present application is directed to devices and methods for formingdevices in which a seed area for heteroepitaxial growth is formed on asemiconductor substrate. The seed area comprises a two-dimensional areawith at least one dimension less than about 100 nm. In an embodiment,the seed area comprises a linear surface dimension that ranges fromabout 10 nm to about 100 nm. In another embodiment, seed areas havingheteroepitaxial structures with linear surface dimensions less than 20or 25 nm can be employed. A heteroepitaxial layer comprising a secondmaterial that is different from the first material can be grown usingthe seed area to nucleate the epitaxial growth. In this application, theterm “linear surface dimension” can refer to any linear dimension of thesurface of the seed area, such as, for example, a width or diameter.

Employing seed areas having relatively small dimensions in the mannerdisclosed herein can have one or more of the following benefits: theability to form heteroepitaxial structures with reduced numbers ofdefects compared with larger area heteroepitaxial layers; the ability toform heteroepitaxial structures with zero or substantially zero defects;the ability to form heteroepitaxial pillar structures that are flexibleand/or that can accommodate strain better than heteroepitaxial layersgrown on a planar substrate surface; or the ability to form small areaheteroepitaxial films that can accommodate strain better thanheteroepitaxial layers grown on a large area of a substrate surface.

The nanoscale heteroepitaxial growth of the present disclosure canexploit the greatly improved materials quality that occurs when thesubstrate, such as, for example a silicon fin, is nanoscale in lateralsize. Thus by using nanoscale heteroepitaxial growth onto, for example,a 10 nm wide silicon area, which is already only ˜20 atoms wide, theheteroepitaxial techniques and devices of the present disclosure cantake advantage of the evolution of integrated circuits. When thedimensions of a growth area are reduced to below the average scale tonucleate a defect such as a threading dislocation, it is possible togrow heterogeneous materials without nucleating either threadingdislocations or antisite defects (boundaries where two grains of thezinc-blende III-V crystal are misoriented by) 180°. The scale for thisdefect free growth is that at least one linear dimension of the growtharea be much less than the mean distance between defects in a large areaheteroepitaxial growth. In an embodiment, dimensions of about 100 nm orless can be employed, such as, for example, about 10 to about 20 nm. Thetable below gives some typical dislocation densities and thecorresponding average distance between dislocations. Note that to besuitable for silicon electronics, the incidence of threadingdislocations that impact the electrical properties of an individualchannel can be exceedingly low. Today's microprocessors contain as manyas 3,000,000,000 transistors, and perhaps as many as 30,000,000,000channels. With continued Moore's law scaling, this number will continueto climb exponentially. The allowed number of defected channels can be avery small fraction of the total number of channels.

Typical Threading Material/ Dislocation Average Distance betweenSubstrate Density (cm⁻²) Dislocations (μm) Ge_(0.23)Si_(0.77)/Si(001) 5× 10⁵ 14 GaAs/Si(001) 1 × 10⁵ 3.2 GaAs/GaAs 1 × 10⁴ 100 InAs/Si(001) 1 ×10⁷ 3.2 GaN/sapphire 1 × 10⁹ 0.32 GaN/SiC 5 × 10⁸ 0.45 GaN/GaN 1 × 10⁶10 GaN/Si(111)  1 × 10¹⁰ 0.1

The prospects for defect-free nanoscale growth are further improved bythe migration of silicon integrated circuits to FinFET architectures. Incontrast to the growth in a simple opening atop a bulk substrate, theFinFET pedestal is significantly more compliant, e.g. it can share thestrain (lattice displacement) associated with the lattice mismatchstress with the growing film. Control of strain in MOSFET channels is animportant aspect of modern integrated circuit manufacturing since thestrain directly impacts the electronic properties of the material.Nanoscale growth provides additional approaches to controlling thisstrain in the FinFET channel by adjusting the dimensions of the “fin”and the thickness and layer structure of the grown material.

FIG. 1A-E illustrate a method for making a heteroepitaxial layer,according to an embodiment of the present disclosure. The methodcomprises providing a substrate 10. Examples of suitable substratesinclude silicon wafers, silicon-on-insulator substrates, or othermaterials used as semiconductor substrates. Suitable substrates arecommercially available and are well known in the art. In an embodiment,the semiconductor substrate comprises silicon having a (001) facetexposed for processing. As an example, the cross-sections of substrate10 shown in the figures herein can have the <110> direction orientedinto the paper.

Nanostructured pedestals 12 are formed on the substrate. TheNanostructured pedestals 12 are comprised of any suitable materialcapable of acting as a seed layer for subsequent epitaxial growth.Examples of suitable materials include doped or undoped single crystalsilicon. Other suitable materials include single crystal III-Vmaterials, such as GaAs and GaSb, which are common substrate materialsin photonics and high-speed electronics; and single crystal GaN,sapphire and SiC. Any other single crystal material that provides asuitable nucleation surface for the desired epitaxial growth can beemployed. In an embodiment, the pedestals 12 can be formed from the samematerial as the substrate, where the substrate is a single crystalmaterial. In other embodiments, the pedestals can be a differentmaterial form the substrate. Any desired technique for forming thesingle crystal pedestals can be used. Examples of such techniquesinclude various methods for patterning and etching the substratesurface. Suitable techniques are well known in the art. For purposes ofstrain relief as discussed below, it is useful to have the pedestalsroughly as high or higher as the smallest in-plane dimension of the seedarea.

Referring to FIG. 1B, a selective growth mask layer 14 is provided on atop surface and side surface of the pedestal. In an embodiment, theselective growth mask layer 14 can be provided over the entire perimeterof the sidewall surface. In an embodiment, selective growth mask layer14 is formed on at least three sides of the pedestal, including the topsurface and front and back sidewalls of a pedestal. Any suitabletechnique for forming the selective growth mask layer, such as oxidationof the semiconductor substrate surface, or deposition of material ontothe surface, can be employed. Suitable methods for forming selectivegrowth masks are well known in the art.

Referring to FIG. 1C, a portion of the selective growth mask layer 14 isremoved to expose at least the top surface 16 of the pedestal 12.Various suitable methods can be employed to selectively remove the topportion of selective growth mask layer 14. Examples includeanisotropically etching to selectively remove the selective growth masklayer from the top surface of the pedestal; or formation of anon-conformal layer, such as by depositing and reflowing a layerfollowed by an etch back process. The etch back process can employpolishing, such as chemical mechanical polishing. Yet other examples oftechniques for removing a portion of selective growth mask layer 14 willbe discussed in greater detail below with respect to the method of FIG.2.

After exposing the top surface of the pedestals 12, the semiconductormaterial of pedestal 12 can optionally be selectively etched back, asshown in FIG. 1D. As a result of the selective etch back, remainingportions of the selective growth mask layer 14 form sidewall barriers18. The sidewall barriers 18 can act to block defects, such as stackingfaults that propagate, for example, along (111) directions (e.g., at anangle to the top surface).

Any suitable process for selectively etching back the pedestal 12 can beemployed. Suitable etch back processes are well known in the art. In anembodiment, the remaining portion of pedestal 12 comprises a (001) facetof silicon material exposed at the pedestal top surface.

Following the selective etch back, an epitaxial layer is grown on theremaining portion of pedestal 12. The exposed top surface of pedestal 12provides a seed area for the epitaxial growth. As described above, theseed area can have at least one dimension that is less than about 100nm. Example configurations for the seed area include a rectangular areahaving with a width dimension ranging from about 10 nm to about 100 nmand a length dimension ranging from about 200 nm to about 2000 nm; or acircular area having a diameter ranging from, for example, about 10 nmto about 100 nm.

In an embodiment, the heteroepitaxial layer comprises a Group III-Vsemiconductor material. Examples of Group III-V semiconductor materialsinclude nitrogen-based materials, such as gallium nitride or other GroupIII-N semiconductors, such as AlGaN, indium nitride (InN), and indiumgallium nitride (In_(x)Ga_(1-x)N). Other examples of Group III-Vsemiconductor materials include InAs and InAsSb, which havesignificantly higher electron mobilities and saturation velocities incomparison with Si. The techniques described also apply to semiconductormaterials other than III-V materials, such as Ge.

The growth of the epitaxial layer is directed by the seed area of thenanostructured pedestal surface. Epitaxial growth copies the underlyingcrystal structure of the substrate, e.g., atoms line up as if they are acontinuation of the starting crystal structure. In the case ofheteroepitaxial growth, the grown film might have the same symmetry asthe seed area, but a different natural distance between atoms (this isthe lattice mismatch mentioned above).

FIG. 1E illustrates an example of a heteroepitaxial nanostructure. Thenanostructure comprises the remaining pedestal 12 having anano-dimensional top surface and a side surface; and the epitaxial layer20 grown on the nano-dimensional top surface of the pedestal. In anembodiment, the remaining selective growth mask layer 14 extends abovethe surface of the pedestal 12 to form sidewall barriers 18. Theepitaxial layer 20 can grow between and above the sidewall barriers 18,at which point the width dimensions of the heteroepitaxial layernanostructure may increase to overlap the insulator, as shown, forexample, in FIG. 1E. Additional growth can result in further enlargementof the heteroepitaxial layer as desired.

FIGS. 2A-2F illustrate a method for making heteroepitaxial layers,according to another embodiment of the present disclosure. Asillustrated in FIG. 2A, the substrate 10 is a silicon substrate.Pedestals 12 are formed by employing an etch mask 22. The etch mask canhave width or diameter dimensions of about 1 micron or less. FIG. 3illustrates a top view, showing the etch mask 22 used to pattern thesubstrate. However, any other suitable shaped pattern can be employed,such as masks for forming square or rectangle features. The etch mask 22can be any suitable mask type, such as a patterned photoresist layer orhard mask formed of, for example, silicon oxide.

Using the etch mask 22, the silicon substrate can be patterned byetching to form the pedestal 12, as illustrated in FIG. 2B. Any suitableetching process can be employed that will form the desired pedestalshape. In an embodiment, a dry etching process is employed.

Following etching, the etch mask 22 can be removed. In an embodiment,thermal oxidation can then be carried out to form a selective growthmask layer 14 of silicon dioxide to a desired thickness. The thermaloxidation process consumes the substrate material, so that the thickerthe silicon dioxide layer, the smaller the resulting width dimension ofthe final pedestal 12. Thus, the thickness of the silicon dioxide can bevaried so that the diameter or width of the silicon pedestal 12 isreduced to any desired size dimension. Example width dimensions can bethe same as those discussed above for FIG. 1. Alternatively, thepedestal 12 can be patterned to the desired final dimension during theetching step; followed by forming a selective growth mask layer todelineate the seed area by a process that does not consume the siliconto reduce the dimensions of the pedestal 12.

Referring to FIG. 2D, a non-conformal layer 24 is formed over thepedestal 12 and selective growth mask layer 14. Any suitable type ofnon-conformal layer can be employed. Examples of suitable non-conformallayers include doped silicon oxides, spin-on-glass, photoresist or othermaterials that can be deposited in liquid form, reflowed, polished orotherwise planarized to reduce surface topography. Suitablenon-conformal layers are well known in the art.

The non-conformal layer can be etched back until the selective growthmask layer 14 at the top of the pedestal 12 is exposed, as illustratedin FIG. 2E. The exposed portion of selective growth mask layer 14 can beremoved by the same or a different etch process as is used to etch backthe non-conformal layer 24. The remaining portion of the non-conformallayer 24 can then be removed if desired, such as where the non-conformallayer is a polymer.

Once exposed, the pedestal top surface can be used without furtherprocessing as a seed area for heteroepitaxial growth, if desired. Duringepitaxy, a single crystal semiconductor grows on the seed area 26 thatis shown exposed in FIG. 2E.

Alternatively, a further selective etch back of the seed material ofpedestal 12 can be carried out to form the sidewall barriers 18 prior toepitaxial growth, as illustrated in FIG. 2F. The etch back of the seedmaterial can be performed by any suitable selective etch process, suchas a dry etching process. Heteroepitaxial growth is then carried outbetween the sidewall barriers 18. Sidewall barriers 18 can block thepropagation of defects, such as stacking faults and misfit dislocations,from the upper region of the heteroepitaxial layer. The resultingstructure is shown in FIG. 4, according to an embodiment of the presentdisclosure.

The epitaxy conditions, such as temperature and the ratio of precursorgases, can be controlled to allow for formation of a planar epitaxiallayer surface. For example, a planar GaN(001) facet at the top of GaNepi-layer can be grown using appropriate growth conditions. One ofordinary skill in the art would be able to determine the desiredconditions without undue experimentation.

The pedestal structures of the present disclosure can provide one ormore of the following benefits: formation of heteroepitaxial materialswith reduced defects; the selective growth mask layer 14 can prevent orreduce nucleation at the pedestal sidewalls, thereby isolating thenucleation during epitaxy to the top facet of the pedestal; pedestalscan provide increased flexibility and/or the silicon pedestal structurecan help relieve strain resulting from the lattice mismatch between thepedestal and the epitaxial material grown thereon.

Still other embodiments are contemplated. FIGS. 5A and 5B illustrate asemiconductor substrate 10 comprising a surface masked with any suitableselective growth mask 50 for patterning the seed areas 26. The mask 50can be relatively thin compared to the thickness of the subsequentlyformed and fully-grown heteroepitaxial layer that is formed on seedareas 26. A similar embodiment is contemplated that employs a thickgrowth mask 50 to prevent lateral overgrowth. Again, the term thick isrelative to the thickness of the fully-grown heteroepitaxial layersubsequently grown on seed areas 26. Examples of suitable growth masksinclude SiO₂, Si₃N₄ or combinations thereof. The dimensions of the seedarea can be any of the seed area dimensions described herein. In anembodiment, seed area dimensions in the substrate plane are ˜10 nm wideby ˜40 nm long. Similar dimensions can be employed for any of the seedareas illustrated in the embodiments of FIGS. 6-9.

FIGS. 6A and 6B illustrate an embodiment comprising semiconductorpedestals, or plateaus, protruding from the substrate 10 to form theseed areas 26. Mask 50 can be any appropriate growth mask such as SiO₂,Si₃N₄ or combinations thereof, that is arranged to be more or less flushwith the top surface of the seed areas 26.

FIGS. 7A and 7B comprise semiconductor pedestals, or plateaus,protruding from the substrate 10 to form the seed areas 26 of thepresent disclosure, according to an embodiment of the presentdisclosure. Mask 50 is formed to be higher than the top of seed areas26.

FIGS. 8A and 8B illustrate an alternative embodiment in which the growthmask 50 is lower than the top of seed areas 26, thereby allowingheteroepitaxial growth on the top and sides of the pedestals. FIGS. 9Aand 9B illustrate an approach in which non-faceted pedestals protrudefrom the substrate, according to an embodiment of the presentdisclosure. The mask 50 can be thinner than the top surface of seedareas 26, thereby allowing growth on the top and sides of the pedestals,similar to the embodiment illustrated in FIG. 8A. In any of theembodiments of FIGS. 5 to 9, the seed areas can comprise any of thesemiconductor materials described for seed areas in the presentdisclosure; and the materials subsequently grown thereon can compriseany of the heteroepitaxial grown semiconductors described herein.Similarly, the mask 50 can be any of the mask materials describedherein.

FIG. 10 illustrates an isometric projection of a FET device 70 that canbe formed using heteroepitaxial layer structures, according to anembodiment of the present disclosure. Pedestal 12 can be formed usingany suitable process, such as with a masking layer to prevent growth onthe sidewalls of the pedestal. In an embodiment, a growth mask such asthat shown in FIG. 7A can be used. The silicon pedestal 12 is shown withsloping sidewalls, but could have any other desired shape.

In an embodiment, heteroepitaxial growth proceeds from the exposedsemiconductor surface, or seed area, of pedestal 12. The seed areasurface can comprise any suitable material, including any seed areamaterials discussed herein. In an embodiment, the seed area surface is aSi(001) surface. An isolation layer 72 can be grown on the seed area.Isolation layer 72 can be, for example, a large bandgap material, toprevent leakage of carriers from the channel into the silicon. As notedabove, isolation layer 72 can also be a layer, such as anAl_(0.98)Ga_(0.02)As layer, that is easily oxidized following the growthto provide additional isolation. An alternative strategy is to dope thesilicon so that it forms a p-n junction with the channel material, alsoreducing leakage of carriers into the silicon. Depending on the detailsof the bandgap alignment between the channel material and the silicon,isolation layer 72 may or may not be necessary.

In an embodiment, it is possible to grow a layer, such as but notrestricted to, a high Al concentration AlGaAs layer, which can beselectively oxidized during the device processing subsequent to growthof the heteroepitaxial layer. This allows epitaxial growth while at thesame time providing the advantages of a semiconductor-on-oxide structurewhere the carriers are strongly confined to the channel. Additionally,the aluminum oxide layer can be selectively removed to provide accessfor a gate-all-around configuration. Examples of this technique aredescribed in U.S. Provisional Patent Application 61/752,741, entitledGate-All-Around Metal-Oxide-Semiconductor Transistors with Gate Oxides,filed Jan. 15, 2013, the disclosure of which is hereby incorporatedherein by reference in its entirety.

Following the growth of the optional isolation layer, a channel layer 74is grown. Channel layer 74 is a heteroepitaxial layer and can compriseany suitable materials discussed herein for heteroepitaxial growth. Inan embodiment, the bandgap engineering that is common in III-V devicescan be used in devices of the present disclosure to, for example, growhigher bandgap cladding layers below and above the active channel layer.This can shield the carriers in the channel from surface defects andreduces scattering and improves carrier mobilities, saturationvelocities, and lifetimes.

In an embodiment, channel layer 74 can comprise several layers. Forexample, channel layer 74 can comprise a GaAs/InGaAs/GaAs structure inwhich the high mobility InGaAs material is clad with upper and lowerhigh bandgap materials to shield the carriers from the higher pointdefect densities at the interface with the pedestal 12 or isolationlayer and at the top surface of the growth.

FIG. 12 shows a possible layered heterostructure sequence for thechannel layer 74, according to an embodiment of the present disclosure.This structure takes advantage of bandgap engineering that is well knownfor III-V devices. The structure comprises a bottom large bandgapmaterial in contact with a pedestal 12. A bottom cladding functions tokeep carriers away from the interface with pedestal 12. A channel regionand a top cladding are formed over the bottom cladding. As is well knownin the art, the channel region and top cladding can optionally berepeated a number of times, as shown, to provide increased currentcarrying capacity if thin channel regions, such as quantum wells, aredesired.

Following the heteroepitaxial growth step of channel layer 74, doping ofthe source and drain regions can be carried out. This can include amasked ion implantation followed by an annealing step to activate theimpurities. This will modify the growth layers by impurity induceddiffusion to lower the resistance of the source-gate-drain transitions.A gate dielectric 76 and source “S”, drain “D” and gate “G” electrodes,as illustrated in FIG. 10, can be formed by any suitable methods.Suitable methods are known in the art.

FIG. 11 shows an embodiment where a dielectric spacer 80 is used toplanarize the structure following the S/G/D metallization. This layercan be thicker than the finFET and contact holes can be created forcontacting the S/G/D, as is well known in the art.

Another geometry of interest is a vertical channel. This embodimentlends itself to a gate all around configuration and has the significantadvantage that the gate is self-aligned to the nanowires. The gatelength can be set by deposition processes which are much morecontrollable than lithography at nm dimensions. Examples of verticalMOSFETS are described in U.S. Pat. No. 8,344,361, the disclosure ofwhich is hereby incorporated by reference in its entirety. The '361patent does not explicitly discuss heterostructure growth from a siliconsubstrate, and is primarily about forming two and three terminaldevices.

FIGS. 13 and 14 illustrate a process for forming high mobility channelvertical transistors based on nanowire growth from nanoscale Si seeds.The seed area can have a linear surface dimension of 100 nm or less,such as 20 or 25 nm or less, or about 10 nm. For specificity, theconcept proceeds in a source down configuration. However, thealternative drain down configuration is also available, as would beunderstood by one of ordinary skill in the art.

As shown, the heteroepitaxial growth starts from Si(001) pedestals 12that can have any desired shape, such as square or round cross-sections,or extended into walls (e.g., a length dimension that is many timeslarger than the width dimension, such as 5, 10 or 100 times or more). Anoptional isolation layer 72 is first grown to isolate the source fromthe Si material. Since the source region 82 is adjacent to the siliconin a source down embodiment, and a good contact can be provided, leakageinto the silicon is not as important to the device performance as it wasfor the horizontal devices where the gate region was in direct contactwith the silicon substrate material. Doping can be varied during theepitaxial growth to provide heavy doping in the source and drain regions82, 84 and reduced doping in the gate region 86.

The de-lineation of the source/gate/drain regions of the nanowiresrefers to doping levels during the growth. The vertical devices areshown in parallel, e.g. all source, gate and drain contacts areconnected to the same metallization. In an actual circuit, only some ofthe devices will be connected in parallel to provide current carryingcapability; other devices would form the channels of differenttransistors in accordance with the circuit design.

Following the growth of the nanowire 88, a dielectric layer 90 isprovided to isolate the silicon followed by formation of the sourcecontact layer 92. Appropriate annealing processes can be employed toassure good contact to the source regions of the nanowires. While all ofthe sources are shown connected in parallel; in practice, one or more ofthe nanowires will be in parallel to provide current carrying capabilityand others will be incorporated into different transistors as dictatedby the circuit design.

Following formation of the source contact 92, a field dielectric layer94 can be deposited to isolate the gate contact 96 from the sourcecontact 92. Initially, the field dielectric layer 94 can stop just shortof the gate region to allow for oxidation of the nanowire to provide thegate dielectric 98. The field dielectric 94 can then be continued to themiddle of the gate region and gate contact 96 is provided.

Following formation of gate contact 96, additional field dielectric 100can be deposited on top of the gate contact to completely cover thenanowires 88. Then an etch back step can be carried out to expose thetop of the drain regions and a drain contact 102 is provided. Additionalprocessing can be used to define the various transistors andinterconnections, as is the case in traditional integrated circuitmanufacturing. There can be many variants on this basic process. Forexample, the gate oxide layer can be removed from the sidewall of thedrain region and contact made using this sidewall in place of the topcontact shown.

Using methods of the present disclosure, growth of the channels and thesource and drain regions of transistor structures can be carried outsimultaneously. An advantage is that the transitions between the source,channel and drain regions are single crystal material thereby providinghigh-quality, low-resistance transitions. Threading dislocations in thesource/drain regions are relatively benign since these are less criticalheavily doped regions, where the electrical impact of the dislocation isreduced by screen associate with the high concentration of carriers.This is true for both horizontal and vertical geometries.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Moreover, all ranges disclosed hereinare to be understood to encompass any and all sub-ranges subsumedtherein.

While the present teachings have been illustrated with respect to one ormore implementations, alterations and/or modifications can be made tothe illustrated examples without departing from the spirit and scope ofthe appended claims. In addition, while a particular feature of thepresent teachings may have been disclosed with respect to only one ofseveral implementations, such feature may be combined with one or moreother features of the other implementations as may be desired andadvantageous for any given or particular function. Furthermore, to theextent that the terms “including,” “includes,” “having,” “has,” “with,”or variants thereof are used in either the detailed description and theclaims, such terms are intended to be inclusive in a manner similar tothe term “comprising.” Further, in the discussion and claims herein, theterm “about” indicates that the value listed may be somewhat altered, aslong as the alteration does not result in nonconformance of the processor structure to the illustrated embodiment. Finally, “exemplary”indicates the description is used as an example, rather than implyingthat it is an ideal.

Other embodiments of the present teachings will be apparent to thoseskilled in the art from consideration of the specification and practiceof the present teachings disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the present teachings being indicated by thefollowing claims.

What is claimed is:
 1. A method for making a heteroepitaxial layer, themethod comprising: providing a semiconductor substrate; forming ananostructured pedestal on the semiconductor substrate, the pedestalhaving a top surface and a side surface; providing a selective growthmask layer on the top surface and side surface of the pedestal; removinga portion of the selective growth mask layer to expose the top surfaceof the pedestal; selectively etching-back the exposed top surface of thepedestal to form a seed area, the seed area having a linear surfacedimension that ranges from about 10 nm to about 100 nm, wherein the seedarea of the pedestal has been selectively etched back; and growing aheteroepitaxial layer on the seed area, wherein the heteroepitaxiallayer is substantially unrestricted by sidewalls during growth andwherein a portion of the heteroepitaxial layer above the selectivegrowth mask layer has a trapezoidal cross-sectional shape proximate tothe seed area.
 2. The method of claim 1, wherein the seed area has beenselectively etched back and is substantially level with the selectivegrowth mask layer.
 3. The method of claim 1, wherein the heteroepitaxiallayer converges toward a point distal to the seed area.
 4. The method ofclaim 1, wherein the top surface of the pedestal forms a seed area thatis approximately coplanar with a top surface of the selective growthmask layer.
 5. The method of claim 1, wherein the heteroepitaxial layeris substantially defect-free.
 6. The method of claim 1, wherein aninterface between the heteroepitaxial layer and the seed area issubstantially defect-free.
 7. The method of claim 1, wherein the entireheteroepitaxial layer is above a top surface of the selective growthmask layer.
 8. The method of claim 1, wherein the semiconductorsubstrate comprises silicon having a [001] direction normal to asubstrate surface.
 9. The method of claim 1, wherein the seed areacomprises a (001) plane of silicon.
 10. The method of claim 1, whereinthe heteroepitaxial layer forms a portion of a transistor.
 11. A methodfor making a heteroepitaxial layer, the method comprising: providing asemiconductor substrate; forming a nanostructured pedestal on thesemiconductor substrate, the pedestal having a top surface and a sidesurface; providing a selective growth mask layer on the top surface andside surface of the pedestal; removing a portion of the selective growthmask layer to expose the top surface of the pedestal; selectivelyetching-back the exposed top surface of the pedestal to form a seedarea, the seed area having a linear surface dimension that ranges fromabout 10 nm to about 50 nm, wherein the seed area has been selectivelyetched back and is substantially level with the selective growth masklayer; and growing a heteroepitaxial layer on the seed area, wherein aportion of the heteroepitaxial layer above the selective growth masklayer has a trapezoidal cross-sectional shape proximate to the seed areaand another portion of the heteroepitaxial layer above the selectivegrowth mask layer converges toward a point distal to the seed area. 12.The method of claim 11, wherein the heteroepitaxial layer issubstantially unrestricted by sidewalls during growth.
 13. The method ofclaim 11, wherein the heteroepitaxial layer is substantiallydefect-free.
 14. The method of claim 11, wherein an interface betweenthe heteroepitaxial layer and the seed area is substantiallydefect-free.
 15. The method of claim 11, wherein the heteroepitaxiallayer slopes outwardly proximate to the seed area.
 16. The method ofclaim 11, wherein the heteroepitaxial layer slopes inwardly distal fromthe seed area.
 17. The method of claim 11, wherein the entireheteroepitaxial layer is above a top surface of the selective growthmask layer.
 18. The method of claim 11, wherein the semiconductorsubstrate comprises silicon having a [001] direction normal to asubstrate surface.
 19. The method of claim 11, wherein the seed areacomprises a (001) plane of silicon.
 20. The method of claim 11, whereinthe heteroepitaxial layer forms a portion of a transistor.
 21. A methodfor making a heteroepitaxial layer, the method comprising: providing asemiconductor substrate; forming a nanostructured pedestal on thesemiconductor substrate, the pedestal having a top surface and a sidesurface; providing a selective growth mask layer on the top surface andside surface of the pedestal; removing a portion of the selective growthmask layer to expose the top surface of the pedestal; selectivelyetching-back the exposed top surface of the pedestal to form a seedarea, the seed area having a linear surface dimension that ranges fromabout 10 nm to about 50 nm, wherein the top surface of the pedestalforms the selectively etched back seed area that is approximatelycoplanar with a top surface of the selective growth mask layer; andgrowing a heteroepitaxial layer on the seed area, wherein theheteroepitaxial layer grows wider than the seed area.
 22. The method ofclaim 21, wherein the heteroepitaxial layer is substantiallyunrestricted by sidewalls during growth.
 23. The method of claim 21,wherein the heteroepitaxial layer slopes outwardly proximate the seedarea.
 24. The method of claim 21, wherein the heteroepitaxial layerslopes inwardly distal from the seed area.
 25. The method of claim 21,wherein the entire heteroepitaxial layer is above a top surface of theselective growth mask layer.
 26. The method of claim 21, wherein aportion of the heteroepitaxial layer above the selective growth masklayer has a trapezoidal cross-sectional shape proximate to the seedarea.
 27. The method of claim 21, wherein another portion of theheteroepitaxial layer above the selective growth mask layer convergestoward a point distal to the seed area.